Reporters without Borders now ranks Mexico as the most risky country in the world for journalists, with nine media professionals murdered in 2016 alone.
He was the fifth journalist (link in Spanish) to be killed in the country so far this year.
Javier Valdez, 50, was shot near the premises of Riodoce, a Mexican news weekly he founded, in his hometown of Culiacan in northwestern Sinaloa state, the source said.
Police later found his vehicle abandoned a few streets away, where it had been left by the gunman, according to the state prosecutor, Juan Jose Rios.
Journalist Javier Valdez was shot and killed when an unknown number of assailants opened fire on his auto, according to the website of online media outlet RioDoce, which he helped found and where he continued to work.More news: An incomplete list of Vladimir Putin's many talents
Images in Mexican media showed a body lying in a street covered by a blue blanket and surrounded by 12 yellow markers of the kind typically used to flag evidence such as bullet casings.
Mexican president Enrique Peña Nieto said Monday afternoon that he has directed the federal police to help local authorities investigate "this outrageous crime".
"I reiterate our commitment to freedom of expression and the press, fundamental for our democracy", he tweeted. "Everybody always deferred to his knowledge", Hootson said.
Valdez was the author of the books "Narcoperiodismo" and "Los Morros del Narco", the latter of which chronicled the lives of young people swept up in Mexico's drug underworld.
But if Valdez's case is anything like that of the many other reporters who have been murdered in Mexico recently, there's a good chance his killers will never be punished.More news: Harish Salve charged Re 1 for fighting Jadhav's case: Swaraj
Filiberto Alvarez was killed in Tlaquiltenango on May 2, Cecilio Pineda Brito was killed in Guerrero on March 2, Ricardo Monlui was killed in Veracruz on March 19, Miroslava Breach was shot dead in Chihuahua on March 23 and Max Rodriguez Palacios was murdered in Baja California Sur on April 14.
"In Mexico a 'war' is raging against journalists", Erika Guevara-Rosas, Americas director at Amnesty International, said in March.
The US-based Committee to Protect Journalists this month said "Mexico's press is caught in a deadly cycle of violence and impunity".
"His loss is a blow to Mexican journalism and to the Mexican public, who see a shadow of silence spreading across the country".
"Even though you may have bulletproofing and bodyguards, (the gangs) will decide what day they are going to kill you". Ricardo Sanchez Perez del Pozo, a lawyer with a background in global law and human rights, took over the post.More news: Three months in, chaos grips Donald Trump's White House
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